The present application relates to safe operation of semiconductor power switching devices and, more particularly, to a novel circuit for preventing excess power dissipation and/or latch-up in power switching semiconductor devices by removal of the drive signal to the control electrode thereof if the drive signal is insufficient to support the device load current.
It is known that a solid-state switching device, particularly when used to control one or more loads operating from the power line and from "starved" power supplies, may not have a control electrode drive signal of adequate magnitude furnished thereto for supporting the larger load current required during a "cold start" of the associated load. For example, in the case of a tungsten load, such as a light bulb and the like, the temperature coefficient of the load results in the start-up load current being considerably greater than the load current drawn by the load once the load has reached steady-state operating conditions. A switching semiconductor device in series with that load may be provided with adequate drive to support the steady-state load current, but not for the larger "cold-start" current, in which case the device operating point moves into the active region and excessive power dissipation occurs. Further, in other instances where a power switching semiconductor device is to be normally operated as either a fully cut-off device or a fully saturated device, transient conditions during normal operation may cause the switching semiconductor device to pull out of the saturation condition into the "active" condition. Operation in the high-dissipation active region often causes the switching device maximum temperature to be exceeded, and the device to be destroyed. The problem is further exacerbated in the case of metal-oxide-semiconductor (MOS) gated devices, particularly devices such as the insulated-gate rectifier, which can enter into a destructive latch-up mode of operation if operated in the active region. It is therefore highly desirable to provide a circuit for controlling a power switching semiconductor device into its cut-off condition if the drive signal at the device control electrode, e.g. the gate electrode in an FET or IGR device, or the base electrode in a power transistor, is not sufficient to maintain the semiconductor device in the saturated condition for an increase in the load current controlled thereby.